Silicon carbide electrical heating elements are known to be susceptible to oxidation which substantially changes their electrical properties. US Pat. No. 3,875,477 (Fredrikkson I) discloses an igniter whose porous recrystallized silicon carbide has an internal network of open porosity. This conventional igniter is produced by forming an igniter-shaped green body comprising fine and coarse SiC particles and firing this green body at about 2400xc2x0 C. in nitrogen. During the firing step, the highly reactive fine SiC particles vaporize and then redeposit on the coarse SiC particles, thereby forming a layer of xe2x80x9crecrystallizedxe2x80x9d SiC which both coats and connects the coarse SiC particles. An example of this conventional recrystallized structure is shown in FIG. 1.
During use, the oxidizing atmosphere surrounding this igniter penetrates the igniter""s porosity, the silicon carbide at the surface of these pores reacts with the oxygen to form silica, an electrical insulator, thereby decreasing the conductive cross-section of the SiC igniter, resulting in decreased amperage and an increased resistance (at a fixed voltage). This phenomenon is known as xe2x80x9cagingxe2x80x9d. It has been found that the conventional recrystallized SiC igniter ages to such an extent that its resistivity increases over 6-12% after only 6000 hours of cycling (5 minutes on, 5 minutes off) at a service temperature of 1480xc2x0 C.
One proposal for minimizing the effects of oxidative aging in electrically conductive refractory bodies is described in US Pat. No. 4,187,344 (Fredrikkson II). Porous SiC heating elements are coated with silicon nitride and/or silicon oxynitride particles in a liquid slurry and these particles are carried into the pores of the element to a depth of at least 6.4 mm. After the slurry has dried, the article is fired at about 1000xc2x0 C. to fix the silicon oxynitride or silicon nitride particles in place and form a barrier against undue oxidation. However, it was found in practice that water vapor and combustion products detrimentally react with the submicron sized silicon nitride/silicon oxynitride impregnant particles. In addition, it was found that the actual penetration produced by this method was less than 0.5 mm.
In other approaches, refractory silicon carbide elements as described in US Pat. No. 3,492,153 are protected by reacting aluminum vapor with nitrogen gas within the pores of the silicon carbide article to form in-situ aluminum nitride. However, this structure does not have an acceptable life in a gas oven environment because of the reactivity between the water vapor in the gas flame and the aluminum nitride impregnant.
Attempts to make igniters more resistant to oxidative deterioration are also shown in US Pat. Nos. 3,509,072; 3,875,476; 4,120,829; and 4,204,863. As each of these disclosures describes the use of various bonding compositions to improve serviceability, the electrical characteristics of the resulting igniters are materially altered.
The solution to peripheral oxidation proposed by US Pat. No. 4,429,003 (Fredriksson III) comprises coating the porous SiC igniter with a slurry of fine silicon carbide particles. The slurry (which is applied by spraying, painting or vacuum impregnation), reportedly flows substantially through the entire porous phase of the body. The treated article is then subjected to an oxidizing atmosphere to convert the silicon carbide particles to silica. Since the silica molecules occupy more space than the SiC particles that were oxidized, their in-situ formation can seal off the pores from further diffusion of oxygen. However, it was found that the resulting silica layer tended to devitrify and undergo a phase change in use, and so was prone to flaking, thereby exposing the underlying igniter to an oxidizing environment. Other methods of providing a protective silica layer have also produced the problematic flaking.
In addition, it has been found that the methods of coating favored by the prior art (e.g., brushing or vacuum infiltration) fail to fully infiltrate the protective particles fully into the porosity of the conventional SiC igniter.
Sealing the porosity of the conventional SiC igniter with an external layer of CVD SiC has been proposed. However, not only is this method expensive, the resulting coated igniters have been found to display inconsistent aging behavior.
Therefore, there is a need for a porous silicon carbide igniter which is more resistant to aging. In particular, there is a need for a material which will remain within 4% of its original design resistivity over 6000 hours of cycling (5 minutes on, 5 minutes off) at a service temperature of 1480xc2x0 C.
It has been unexpectedly found that infiltrating the open porosity of the conventional recrystallized porous SiC body with fine SiC particles, and then recrystallizing the infiltrated particles to form a second layer of recrystallized SiC produces a new SiC material having superior aging resistance. In particular, this new material was found to have stayed within only about 4% of its original design resistivity over 6000 hours of cycling (5 minutes on, 5 minutes off) at a service temperature of 1480xc2x0 C.
For the purposes of the present invention, the conventional recrystallized porous SiC body will be called the xe2x80x9cfirst firedxe2x80x9d body, and the body produced by subsequent infiltration and recrystallization of the first fired body will be called the xe2x80x9cfinal firedxe2x80x9d or xe2x80x9crefiredxe2x80x9d body.
Without wishing to be tied to a theory, it is believed that the second (or xe2x80x9couterxe2x80x9d) layer of recrystallized SiC decreases the internal porosity of the first fired body, thereby reducing the total surface area available for oxidation. Since the amount of oxidation which occurs in a given body is proportional to the amount of surface area available for oxidation in the body, this reduction porosity thereby reduces the oxidation which takes place in the body and thereby reduces the aging.
In that the added layer of silicon carbide is a semiconductor, the small change in resistivity in the re-fired body observed during life testing is surprising in light of the teachings of the art, which taught that protective coatings applied to resistive ceramics should be electrical insulators which, if oxidized in use, would not alter the overall resistance of the heating element. Simply, the prior art taught that the added silicon carbide layer would likely oxidize and in doing so would change the electrical characteristics of the element.
It has also been found that using sonication to infiltrate the fine silicon carbide particles into the first fired body results in complete impregnation of that body to depths of more than 1 mm, an advantage not realized by the prior art methods of coating, brushing and vacuum infiltration.
Moreover, it has been found that controlling the extent of the initial recrystallization in the first fired body is also critical to achieving the lowest pore volumes in the re-fired body. The present inventors discovered that when the first fired body is not fully recrystallized (i.e., it has more than 10% fine SiC particles identifiable by optical or scanning electron microscopy of polished cross-sections of the first-fired body), those fines clog the internal pathways of the body, thereby preventing more full penetration of that body during the subsequent infiltration step. The inventors found that when the first fired SiC body is essentially fully recrystallized (and preferably has less than 5 wt % identifiable fines), the absence of fine SiC particles allows more full penetration of the body during impregnation, thereby reducing the porosity in the re-fired body. The present inventors have found that requiring the first fired body to be fully recrystallized allows the porosity in the re-fired body to be reduced from about 14-18 vol % to about 9-11 vol % Previously, the lowest porosity achievable was about 14 vol %. The present inventors have found that firing to fully recrystallize the fines of the green body can be achieved by firing in nitrogen at times and temperatures sufficient to achieve full recrystallization, or in argon at lower times and temperatures. However, the present inventors have found that performing the first firing step in nitrogen allows for better control of the electrical characteristics of the re-fired body. Other methods of providing full recrystallization may include:
a) firing the material in an atmosphere which increases the surface free energy of the SiC material (i.e., does not provide dangling bond caps), thereby increasing the SiC""s reactivity,
b) reducing the average grain size of the SiC material in order to increase the surface free energy. This can be done by, for example, either decreasing the average size of the fine fraction or by increasing the fraction of fine grains, and
c) infiltrating the internal porosity of the SiC body with a material in which SiC is sufficiently soluble and which also reduces the surface free energy of the SiC, thereby providing for easier dissolution of the fine SiC particles and providing a means for their transport to the coarse SiC grains.
Lastly, the present inventors found that the second recrystallization step undesirably decreased the nitrogen level in the re-fired body, thereby undesirably decreasing the high temperature resistivity of the re-fired body. It was found that adding an aluminum source to either the green body or the impregnation slurry can effectively raise the amount of nitrogen accepted by the first fired body to such a level that the subsequent reduction in nitrogen experienced during the second firing results in the desired amount of nitrogen in the re-fired body.
Therefore, in accordance with the present invention, there is provided a SiC body comprising (and preferably, consisting essentially of):
a) at least 30 wt % coarse silicon carbide particles having a particle size of at least 30 um, and
b) a coating of recrystallized alpha silicon carbide which coats and connects the coarse silicon carbide particles throughout the body,
wherein the coarse silicon carbide particles and the coating comprise at least 89 vol % of the body.
Preferably, the body further comprises less than 2 wt % free silicon, more preferably less than 0.5 wt %. Also preferably, the coating comprises:
a) an intermediate layer of recrystallized alpha silicon carbide which coats and connects the coarse silicon carbide particles throughout the body, and
b) an outer layer of recrystallized alpha silicon carbide which coats the intermediate layer of recrystallized silicon carbide.
Preferably, the intermediate SiC layer has less than 10% (more preferably, less than 5 wt %) identifiable fine SiC particles per unit weight of the intermediate recrystallized layer. In preferred embodiments, the outer layer of recrystallized SiC is present throughout the body, and the porosity of the body is between 8 vol % and 10 vol %.
Also in accordance with the present invention, there is provided a process for making an oxidation-resistant SiC body, comprising the steps of:
a) forming a green body comprising fine and coarse SiC particles (preferably further comprising aluminum-containing particles),
b) firing the green body to form a recrystallized first-fired SiC body (preferably in nitrogen at a time and temperature sufficient to fully recrystallize the fine SiC particles to form an intermediate recrystallized layer, wherein the intermediate SiC layer has less than 10% identifiable fine SiC particles per unit weight of the intermediate recrystallized layer,
c) infiltrating (preferably by sonication) the first fired body with a slurry comprising SiC particles (preferably having a particle size of between 0.2 um and 5 um, more preferably between 0.5 and 3 um, and preferably comprising at least 50 wt % of the slurry) to obtain an impregnated body, and
d) firing the impregnated body in a non-oxidizing atmosphere (preferably in nitrogen above 2200xc2x0 C.) to obtain a re-fired body (preferably, having a porosity of less than 11 vol %).